Parameters and surrounding rock control of gob-side driving under double key stratum after roof cutting

Taking the return-airway 4204 with roof cutting in Longquan Coal Mine as the engineering background, roof structure, key parameters, and deviatoric stress evolution were studied. Conclusion: The Key Stratum within a 4–8 times mining height is considered as Near Key Stratum. Cutting the roof makes it possible to form a cantilever structure of the Key Stratum on the solid coal side, which is more conducive to the stability of gob-side roadway. During cutting angle of 90–55°, the deviatoric stress increases linearly, and the increase rate is coal pillar > solid coal > roof > floor. While cutting length from 0 to 35 m, the deviatoric stress decreases linearly, and the decreasing range: coal pillar > solid coal > roof > floor. When coal pillar width is from 30 to 4 m, the deviatoric stress of left side and floor presents a “single peak” distribution. The deviatoric stress of coal pillar changes from an asymmetric “double peak” to a bell-shaped distribution, and the deviatoric stress of roof changes from a “single peak” to an asymmetric “double peak” distribution. Under same coal pillar width, the deviatoric stress of left, coal pillar and roof after roof cutting decreases most obviously, followed by the floor. Finally, the coal pillar width is 8 m, the cutting angle is 75°, the cutting length is 20 m, and the hole spacing is 1.0 m. The support scheme is bolt + metal mesh + steel belt + anchor cable combined support. The stable period of roadway is about 10 days.


Mine location
The Longquan Coal Mine is situated in Loufan County, Shanxi Province, belonging to the southern area of the Ningwu coalfield.The geological condition of Longquan Coal Mine is simple, and the main coal-bearing strata are the Shanxi Formation of the Lower Permian and the Taiyuan Formation of the Upper Carboniferous.Currently.4#coal seam is mainly mined.

Geological conditions and mining relationship
The 4# coal seam has a range of thickness from 6.2 to 7.4 m, and is usually 6.8 m thick (with a buried depth of around 455 m), so the entirety of the area is retrievable and secure.4# coal seam is gas coal.The coal is black and glassy, with a brown streak, and a bright fissure.
The distribution of the rock stratum is shown in Fig. 1.The immediate roof is composed of sandy mudstone with a thickness of 0.5-2 m, with an average thickness of 1.63 m.It is deep gray with horizontal bedding and contains fossilized plant leaves.There are two Key Stratum from the bottom to the top.The medium sandstone (Key Stratum I) is 4.50-7.62m thick, with an average thickness of 6.06 m, grey-white, and mainly quartz.Fine sandstone (Key Stratum II) is 4.5-9 m thick, with an average thickness of 5 m.5.5 m-thick carbonaceous mudstone is between the two Key Stratum.
The immediate floor is sandy mudstone with a thickness of 3-15.1 m and an average thickness of 5.1 m, containing scattered pyrite and plant leaf fossils and joint development.The hard floor is 5.09 m fine sandstone; the thickness of siltstone and sandy mudstone interbedding is 7.40-12.00m; the average thickness is 9.70 m, greyish black, calcareous cementation, hard.
As shown in Fig. 2, after the 4110-working face (250 m wide) is mined out, it is planned to arrange the return airway of the 4204-working face (250 m wide) along the gob.As the top coal is broken, the return-airway 4204 is planned to be arranged along the roof.The mining method of the two working faces is top coal mining at all height at one time, where the coal cutting height is 3.5 m, and the coal caving height is 2.7-3.9 m.

Relationship between Key Stratum and caving zone height
According to Fig. 1, the medium sandstone (Key Stratum I) is located between 1.63 m and 7.69 m above the coal seam, and the fine sandstone (Key Stratum II) is located between 13.19 and 18.19 m above the coal seam. (1)

Roof cutting length and double cantilever structure
When the roof cutting line only cuts through the medium sandstone (Key Stratum I) as depicted in Fig. 4a, the medium sandstone (Key Stratum I) forms a cantilever structure rock plate A 1 .Under the influence of gravity, the Key Stratum I at the gob side is broken into multiple pieces.The rock block C 1 n is superimposed on the rock plate A 1 .At this point, the fine sandstone (Key Stratum II) is not cut off and retains its traditional breaking pattern, resulting in the formation of A 2 , B 2 (arc triangle block), and C 2 .The arc triangle block B 2 forms a movable three-hinge arch structure with rock plate A 2 and rock block C 2 .Then, the behaviour of the mine pressure in the gob-side roadway is regulated by the arc triangle block B 2 .It is clear that this structure will continue to have an impact on the lower coal seam, and it is not conducive to the stability of the gob-side roadway.
When the roof cutting line cuts through both the medium sandstone (Key Stratum I) and the fine sandstone (Key Stratum II), the breaking pattern of these two Key Stratums is completely altered, as seen in Fig. 4b.The fine sandstone (Key Stratum II) is transformed into a cantilever structural plate A 2 , and the rock block C 2 n overlaps with the rock plate A 2 .Under the influence of gravity, it can smoothly slide towards the gob.The roof cutting altered the failure mechanism of the double Key Stratum, causing the absence of arc triangle blocks and resulting in a cantilever structure for the double Key Stratum.The rock slab that was cut at the gob side will slide down into the gob under the force of gravity.As a result, return-airway 4204 (gob-side roadway) is positioned

Analysis of the cutting angle and S-R stability principle of composite rock block C n
The rock blocks C 1 n and C 2 n hold the weak middle layer (mudstone) together, forming the composite rock block C n .It forms a hinged relationship with the rock plates A 1 and A 2 .To better utilize the mine pressure to make the composite rock block C n slide and avoid the underlying strata bearing more rock weight, it is necessary to design the cutting angle reasonably.
Without considering the failure of the roof-cutting surface caused by a slip, it is considered that the internal friction angle of the roof-cutting surface is a constant value.According to Mohr-Coulomb theory and the S-R stability principle.The stable relationship between composite rock block C n and rock slabs A 1 and A 2 is shown in Fig. 5.The friction force on a roof-cutting surface is the product of the normal stress on that surface and the coefficient of friction (the tangent value of cohesion), which helps prevent the block from sliding.It acts as a counterforce with the shear stress on the roof cutting surface, as shown in Eq. ( 2).
where T CnA is the horizontal force of the composite rock block C n acting on the cutting surface; R CnA is the vertical force of the composite rock block C n acting on the cutting surface; θ is the angle between the cutting surface and the vertical direction (55-90°); and φ is internal friction angle (38-45°).
(2)  When the left side of Eq. ( 3) is larger than the right side, the composite block C n slides; when the left side is equal to the right side, the composite block C n is in a critical state; when the left side is smaller than the right side, the composite block C n is stable.When the roof cutting angle is closer to 90° (the roof cutting line is perpendicular to the roof), the smaller the value of the right side of Eq. (3), the smaller the vertical load force R CnA of the composite block on the cutting surface, the smaller the normal stress component R CnA cosθ + T CnA sinθ on the roof cutting surface, and the smaller the generated friction force.At this point, it is most beneficial for the composite block C n to slide down.
In combination with the analysis in Sect."Relationship between Key Stratum and caving zone height", it can be observed that the roof-cutting line has a slight inclination towards the solid coal side, which favours the formation of an "inverted trapezoidal" roof-cutting structure for the roadway roof.Considering the scheme in which the roof cutting line slightly inclines to the solid coal side, it can not only promote the sliding of the composite block C n after mining, but also ensure the stability of the roadway before mining.

Model parameters
The numerical model is established with 3DEC 5.2.The dimensions of the model are: 100 m (length) × 5 m (width) × 64.88 m (height).The bottom, front, back, left, and right boundaries of the model are displacement boundaries, and the top is a stress boundary (with an equal weight of rock).The Mohr-Coulomb model was adopted for the rock block, and the Coulomb slip model was adopted for the joint.Based on the test results of the basic mechanical properties of the surrounding rock, the parameters of each stratum and contact surface are determined, as shown in Table 1 and Table 2.

Simulation scheme
Rectangular section of the return-airway 4204: width × height: 5.5 m × 3.8 m.The parameters of the anchor bolt and anchor cable are presented in Table 3.
The key parameters of roof cutting include roof cutting angle, cutting length, and different coal pillar widths (roof cutting or no cutting).Other simulation schemes are outlined in Table 4, Table 5, and Table 6.There are a total of 38 models.This section only lists this one numerical model, as shown in Fig. 6.

Physical meaning of deviatoric stress
In China, abutment pressure is commonly used as an indicator of mine pressure, however, it does not take into account the relationship among various stresses.It is the alteration of the relationship among stresses that leads to the failure of rocks.Is there a physical quantity that can reflect both the stress state of the rock and the relationship between various forces?Deviatoric stress can effectively reconcile the above two factors.
The stress state at any point in a rock medium is illustrated in Fig. 7.It can be decomposed into two parts: the spherical stress tensor and the deviatoric stress tensor.The mathematical expressions for these two tensors are given.8b) and shape change (as shown in Fig. 8c).The spherical stress tensor is responsible for changes in volume, while the deviatoric stress tensor causes changes in the material's shape.These shape changes ultimately led to the deformation and failure of the rock (as shown in Fig. 8 a).
In Eq. ( 4), σ m is the spherical stress, calculated as σ m = (σ 1 + σ 2 + σ 3 )/3 in MPa; S i represents the main deviatoric stress, calculated as S i = σ i -σ m in MPa.Among them, the first principal deviatoric stress S 1 plays a significant role in the distortion and fracture of rock, and is often referred to as deviatoric stress.It is the chosen measurement index in this study.

Deviatoric stress of the gob-side roadway affected by cutting angle
According to the simulation scheme in Table 4, the fixed coal pillar width is 8 m, and the roof cutting length is 20 m.The influence of different cutting angles (90°-55°) on the deviatoric stress of the gob-side roadway is discussed. (4)

Deviatoric stress distribution influenced by cutting angle
As illustrated in Fig. 9, the deviatoric stress is observed to be concentrated primarily at the head of the roofcutting line and subsequently transferred to deeper regions within the rock stratum.The closer the roof-cutting line is to the side of the roadway (return-airway 4204), the more pronounced the stress concentration in the surrounding rock becomes.From the perspective of the surrounding rock structure, the smaller the cutting angle, the less sufficient the collapse of the key rock stratums in the mined area, resulting in an increase in the weight of the overlying rock layers transmitted to the underlying rock layers, and subsequently leading to an increase in the degree of deviatoric stress concentration in the underlying coal layers.
Extract the deviatoric stress values on the roof, floor, and sides of the survey line, as represented in Fig. 10.As the cutting angle is decreased from 90 to 55°, the deviatoric stress on the left side of the roadway (solid coal) and the floor shows a single peak distribution, with the peak located at approximately 2 m.The deviatoric stress on the right side of the roadway (coal pillar) shows a bell-shaped distribution, with the peak value located approximately in the middle of the coal pillar.The deviatoric stress on the roof shows an asymmetric double-peak distribution, with the shallow peak located at approximately 2 m depth and the deeper peak located at approximately  www.nature.com/scientificreports/14 m depth.The deeper peak is greater than the shallow peak.As the cutting angle decreases, the peak values of deviatoric stress gradually increase, but there is no obvious change in the peak position.

Vertical comparison of cutting angle effect on the deviatoric stress peak value
To determine the effect of the cutting angle on the maximum deviatoric stress, the maximum values of the deviatoric stress should be compared with that of a cutting angle of 90° and calculated using the Eq. ( 5).
The results can be found in Table 7 and Fig. 11.When the cutting angle is 90°, the peak values of deviatoric stress on the left side, right side (coal pillar), roof, and floor of the roadway are 8.31 MPa, 11.61 MPa, 4.76 MPa, and 6.06 MPa, respectively.When the cutting angle is 75°, the peak values of deviatoric stress on the left side, right side (coal pillar), roof, and floor of the roadway are 9.98 MPa, 14.10 MPa, 5.33 MPa, and 6.16 MPa, respectively, which are an increase of 20.09%, 21.44%, 12.10%, and 1.62% respectively compared to the case when the  As analyzed above, it can be inferred that the best unloading effect is achieved when the cutting angle is 90° (perpendicular to the roof).During the cutting angle range of 90-55°, the peak value consistently exhibits the characteristic of the right side (coal pillar) > left side (solid coal) > floor > roof.As a result of decreasing the cutting angle, the transfer of weight from the rock layer downwards is intensified, leading to a concomitant elevation in the magnitude of stress concentration and peak values of the underlying rock stratum.From the perspective of amplification, the characteristics are right side (coal pillar) > left side (solid coal) > roof > floor.This implies that increasing the cutting angle can effectively improve the stress environment of the lower gob-side roadway, with the most significant improvement observed on the right side (coal pillar) and left side (solid coal) side.

Deviatoric stress distribution influenced by cutting length
Based on the simulation scenario presented in Table 5, with a fixed cutting angle of 75° and coal pillar width of 8 m, the evolution of the deviatoric stress in the surrounding rock of the gob-side roadway (return-airway 4204) is studied when the cutting length varies from 0 to 35 m.
When the cutting length is 5-10 m, only the immediate roof and the medium sandstone (Key Stratum I) are cut off.Fine sandstone (Key Stratum II) can still form a hinged structure at the edge of the gob, which can transfer the weight of the overlying strata to the underlying coal seam.When the cutting length is greater than 20 m, the fine sandstone (Key Stratum II) is also cut off.At this point, the double Key Stratum on the solid coal side becomes a double cantilever structure.The increase in cutting length leads to a more complete collapse of the rock layers on the gob side, as illustrated in Fig. 12, which manifests itself as a reduction in the degree of stress concentration of the deviatoric stress.The deviatoric stress is concentrated at the head of the cutting line.With the increase of the cutting length, the deviatoric stress is gradually transferred to the deeper rock layers.
Extract the deviatoric stress on the roof, floor, and both sides of the survey line, as shown in Fig. 13, and its distribution is similar to those of the cutting angle in the previous section.The details are as follows: In the process of cutting the roof from 0 to 35 m, the deviatoric stress on the left side (solid coal) and the floor of the roadway both show a "single peak" distribution, and the peak position is around 2 m deep.The deviatoric stress on the right side of the roadway (coal pillar) is bell-shaped, and the peak value is approximately located in the middle of the coal pillar.The peak value of deviatoric stress on the roof shows an asymmetric "double-peak" distribution, with the shallow peak located around 2 m deep in the roof and the deep peak located around 14 m deep in the roof, with the deep peak being larger than the shallow peak.As the length of the roof cutting increases, the peak values of the deviatoric stress gradually decrease, but the position of the peak values does not change significantly.

Vertical comparison of cutting length effect on the deviatoric stress peak value
To quantify the effect of the cutting length on the peak value of deviatoric stress, the peak values are compared against the deviatoric stress of the cutting length of 0 m, respectively, and calculated using the Eq. ( 6).The results are shown in Table 8 and Fig. 14.When the roof cutting length is 0 m, the deviatoric stress peaks on the left side, right side (coal pillar), roof, and floor of the roadway are 10.68 MPa, 17.13 MPa, 5.73 MPa, and 6.29 MPa, respectively.When the cutting length is 20 m (the cutting line passes through the Key Stratum II), the peak values of deviatoric stress on the left side, right side (coal pillar), roof, and floor of the roadway are 9.98 MPa, 14.10 MPa, 5.33 MPa, and 6.16 MPa respectively.These values represent reductions of 6.58%, 17.66%, 6.98%, and 2.12%, respectively, compared to the deviatoric stress peaks at a cutting length of 0 m.When the cutting length is 35 m, the peak values of deviatoric stress on the left, right side (coal pillar), roof, and floor are 7.77 MPa, 10.81 MPa, 5.11 MPa, and 6.02 MPa respectively.Compared to the deviatoric stress peaks at a cutting length of 0 m, these values represent reductions of 27.32%, 36.86%,10.87%, and 4.21% respectively, as shown in Fig. 14.The deviatoric stress peak values decrease in a roughly linear manner.
From the analysis above, it can be seen that the longer the cutting length is, the better the pressure relief effect is.When the cutting line is less than 15 m (before crossing the Key Stratum II), the peak value of deviatoric stress decreases gradually.When the cutting length is greater than 15 m, the rate of decrease in the peak value of deviatoric stress increased significantly.As the cutting length reaches 25 m, the rate of decrease in the peak deviatoric stress slows down.In the process of cutting length of 0-35 m, from the peak value, the peak values consistently display the characteristics of the right side (coal pillar) > left side (solid coal) > floor > roof.The increase of the cutting length cuts off the hanging plates of the two Key Stratum at the gob side, and the degree of deviatoric stress concentration of the lower strata was reduced.From the perspective of amplitude reduction, it is characterized by the right side (coal pillar) > left side (solid coal) > roof > floor.This shows that reasonable cutting length can effectively improve the stress environment of the lower gob-side roadway, and the most obvious improvement effect is on the right side (coal pillar) and left side (solid coal).

Deviatoric stress distribution influenced by pillar width
Table 6 simulation scheme is adopted to study the evolution of deviatoric stress of gob-side roadway with different pillar widths under the conditions of no roof cutting and roof cutting (length 20 m, angle 75°).As depicted in Fig. 15a-d, without roof cutting, when the previous working face ( 4110) is mined out, the overlying medium sandstone (Key Stratum I) and fine sandstone (Key Stratum II) form a hinge structure at the edge of the gob, supporting the overlying rock stratum while transferring stress downward.The wider the coal pillar, the further the gob is from the gob-side roadway (return-airway 4204), and the lower the deviatoric stress of the surrounding rock of the gob-side roadway (return-airway 4204).However, this improvement in stress conditions along the gob-side roadway (return-airway 4204) comes at the cost of increasing the width of the coal pillar.
When the previous working face (4110) is mined and the roof is cut, the caving mode of the two Key Stratum is changed, and leading to a concentration of stress at the head of the cutting line which is subsequently transferred to the deeper zone of the stratum.As illustrated in Fig. 15e-h, the collapse of the strata on the gob side is more pronounced, thus reducing the proportion of the weight of the overlying strata transmitted downwards through the articulated structure.Through vertical comparison, it can be observed that when the coal pillar width is the same, the concentration degree of deviatoric stress under roof-cutting conditions is significantly lower than that under non-roof-cutting conditions.For example, when the width of the coal pillar is 16 m, the concentration degree of the deviatoric stress in the surrounding rock of the roadway under the condition of roof cutting (Fig. 15b) is significantly lower than that under the condition of no roof cutting (Fig. 15f).
The deviatoric stress of the survey line, as shown in Fig. 16a,b, is similar in distribution patterns when the roof is not cut and when it is cut.
The distribution of left side (solid coal) deviatoric stress is "single peak" shaped, and as the width of the coal pillar decreases from 30 to 4 m, the position of the peak deviatoric stress on the left side does not change significantly, remaining around 2.0 m in depth, but the peak value gradually increases.
The deviatoric stress distribution of coal pillar: when the width of the coal pillar is greater than 10 m, the deviatoric stress is in the shape of an "asymmetric saddle", which shows that the deviatoric stress on the gob side is higher than that on roadway side.When the width of the coal pillar is 10-4 m, the deviatoric stress of the coal pillar is bell-shaped, and the peak value is roughly located in the middle of the coal pillar.As the width of the coal pillar decreases, the peak value of deviatoric stress first increases and then decreases.
The distribution of roof deviatoric stress: When the coal pillar width is greater than 10 m, the deviatoric stress is approximately in the shape of a "single peak, " with the peak of deviatoric stress located around 2 m deep in the roof.As the width of the coal pillar decreases from 10 to 4 m, the deviatoric stress in the deep part of the roof sharply increases and gradually transits to an asymmetric "double peak" shape.
The distribution of floor deviatoric stress: the deviatoric stress is roughly in the shape of a "single peak, " with the peak of deviatoric stress around 2 m deep in the floor.As the width of the coal pillar decreases from 35 to 4 m, the distribution of shear stress does not change significantly.The position of the peak remains relatively stable.

Vertical comparison of pillar width effect on the deviatoric stress peak value
With the decrease in coal pillar width, it is impossible to share more deviatoric stress caused by strata movement, and the gob-side roadway (return-airway 4204) is closer to the gob.To quantify the influence of coal pillar width on the peak value of deviatoric stress, the peak values of deviatoric stress are compared with that of coal pillar width of 30 m, respectively, and calculated with the Eq. ( 7).
The results are shown in Table 9, Table 10 and Fig. 17.When the width of the coal pillar is from 30 to 4 m, the deviatoric stress of the surrounding rock increases.From the perspective of peak development, the left side and floor increase in a linear way; the right side (coal pillar) increases first and then decreases, and the roof increases in a jumping way.In terms of growth rate, when the roof is not cut off, the peak value of deviatoric stress of the left side increases by 46.91%, that of the right side (coal pillar) by 48.00%, that of the roof by 100.50%, and that of the floor by 41.40%.After roof cutting, the peak value of the deviatoric stress of the left side increased by 0-17.22%, that of the right side (coal pillar) increased by 0-79.91%, that of the roof increased by 0-118.15%,and that of the floor increased by 0-39.49%.Regardless of whether the roof is cut off or not, the reduction of the coal pillar width will seriously affect the deviatoric stress in the surrounding rock of the gob-side roadway, especially the roof and the right side (coal pillar).

Horizontal comparison of deviatoric stress peak value with no roof cutting and roof cutting
After cutting, the deviatoric stress of the surrounding rock is reduced.To quantitatively evaluate the reduction under the same coal pillar width, Eq. ( 8) is used for calculation.
The results are presented in Table 11.From the range of reduction, the peak value of deviatoric stress on the left side is reduced by 4.09-27.19%,that on the right side (coal pillar) is reduced by 11.48-32.26%,that on the roof is reduced by 6.98-16.46%,and that on the floor is reduced by 0.11-4.07%.It can be observed that under the same width of a coal pillar, the deviatoric stress peak values of the left side (solid coal), right side (coal pillar), and roof are reduced significantly after roof cutting, while the deviatoric stress peak value of the floor only reduces by a few percent.Therefore, it can be deduced that roof cutting can significantly reduce the peak values of deviatoric stress in the gob-side roadway (return-airway 4204), particularly for the left side (solid coal), right side (coal pillar), and roof.

Limitations
(1) The successful sliding of composite rock block C n is controlled by factors such as the thickness, length, and width of the Key Stratum after its fracture, but a detailed analysis of these factors is not provided.(2) The cutting length and the cutting angle are two related factors.When the cutting angle decreases and the cutting length remains the same, the cutting height will decrease, which is not elaborated on in this paper.

Conclusions
(1) When the cutting angle is decreased, the deviatoric stress increases, the increase rate: right side (coal pillar) > left side (solid coal) > roof > floor.A slight decrease in the cutting angle, the "inverted trapezoidal" can be placed on the roadway side, preventing the roof from collapsing prematurely, and 75° is selected as the cutting angle.(2) The increase of the cutting length reduces the deviatoric stress, especially when the cutting length (> 20 m) exceeds Key Stratum II, with a significant decrease in deviatoric stress peak value, with characteristics of the right side (coal pillar) > left side (solid coal) > roof > floor.(3) During the process of coal pillar width decreasing from 30 to 4 m, the deviatoric stress distribution of the left side and the floor is in a "single peak" shape, whereas the deviatoric stress distribution of the right side (coal pillar) undergoes a transformation from an asymmetric "double peak" shape to a "bell-shaped".At the same time, the deviatoric stress distribution of the roof changes from a "single peak" shape to an asymmetric "double peak" shape.(4) Under the same coal pillar width conditions, after roof cutting, the deviatoric stress peak value of the left side (solid coal), right side (coal pillar), and roof decreases significantly, by about 20%, and the peak value of the floor decreases by only a few percent.(5) Accumulative blasting can efficiently complete the artificial cutting of the roof, with a blasting hole spacing not greater than twice the radius of the blasting crack damage, which can be adjusted according to the on-site rock properties.

Figure 3 .
Figure 3. Different positions of roof cutting line.

Figure 4 .
Figure 4.The rock structure of gob-side roadway with different cutting lengths: (a) Cutting off Key Stratum I; and (b) Cutting off Key Stratum I and II.

Figure 5 .
Figure 5. Cutting surface stability model of S-R theory: (a) cutting surface force diagram; and (b) mechanical equilibrium decomposition diagram of cutting surface.

Figure 6 .
Figure 6.Numerical model of a gob-side roadway with cutting angle 75°, cutting length 20 m, coal pillar width 8 m.

Figure 7 .
Figure 7. Stress state decomposition at any point.

Figure 8 .
Figure 8. Decomposition of deformation.(a) changes in volume and shape; (b) changes in volume and (c) changes in shape.

Figure 13 .
Figure 13.Deviatoric stress distribution of gob-side roadway with different cutting lengths.

Figure 14 .
Figure 14.The reduction of deviatoric stress peak value affected by cutting length.
Gray-white, mainly quartz, argillaceous cementation, the layer contains a large number of carbon particles, the bottom of the coal infection into black Dark gray sandy mudstone, horizontal bedding, layer containing mirror coal, yellow iron thin layer, with joints Black gray mudstone, massive, containing scattered pyrite and plant leaf fossils, joint development Upper part is gray black, mainly quartz.The middle part is black sandy mudstone, medium fine sandstone, siliceous cementation Grayish black, calcareous cementation.The sandy mudstone below 606.90 m contains two coal lines and a large number of rhizome fossils

Table 1 .
Mechanics parameters of each stratum.

Table 2 .
Mechanics parameters of the joint surface.

Table 3 .
Parameters of the anchor bolt and anchor cable in 3DEC.

Table 4 .
Simulation scheme of different cutting angles.

Table 5 .
Simulation scheme of different cutting lengths.

Table 6 .
Simulation scheme of different coal pillar widths.

Table 7 .
Comparison of the deviatoric stress peak values of gob-side roadway with different cutting angles.cuttingangle is 90 degrees.When the cutting angle is 55°, the peak deviatoric stress of the left side, the right side (coal pillar), the roof, and the floor are 10.27MPa, 17.21 MPa, 5.77 MPa, and 6.44 MPa respectively.The peak deviatoric stress of the left side, the right side (coal pillar), the roof and the floor are increased by 23.54%, 48.22%, 21.22%, and 6.40% respectively, as shown in Fig.11.The growth is roughly linear.

Table 8 .
Comparison of the deviatoric stress peak values of gob-side roadway with different cutting lengths.

length/m Deviatoric stress of left side (solid coal) Deviatoric stress of right side (coal pillar) Deviatoric stress of roof Deviatoric stress of floor Peak
value /MPa Amplification/% Peak value/MPa Amplification/% Peak value /MPa Amplification/% Peak value/MPa Amplification/%

Table 9 .
Comparison of the deviatoric stress peak values of gob-side roadway with different pillar widths without cutting.

Table 10 .
Comparison of the deviatoric stress peak values with different pillar widths after roof cutting.

and key parameters of the gob-side roadway by roof cutting and surrounding rock Roof cutting control principle of the gob-side roadway
Figure 17.Variation of deviatoric stress peak value of gob-side roadway with different pillar widths.
According to the traditional theory of gob-side entry driving in China, after the previous working face (4110) is mined, the main roof collapses and forms three blocks A, B (arc triangle block), and C. The gob-side entry driving is carried out beneath the arc triangle block B, and the blocks B, A, and C form a three-hinged arch structure.The stability of the arc triangle block B plays a crucial role in determining the stability of the lower gob-side roadway.

Table 11 .
Horizontal comparison of the deviatoric stress peak value of gob-side roadway with different coal pillar widths without roof cutting or roof cutting.